1. Field of the Invention
The present invention relates to an electrolyte-electrode assembly, preferably usable for a fuel cell, which comprises an electrolyte interposed between an anode and a cathode, and a method for producing the same.
2. Description of the Related Art
As concern over environmental protection is increasing, fuel cells, which have low-pollution electric power supply sources, are attracting attention. In recent years, attempts have been made to adopt an oxide ion (O2−) conductor as the electrolyte for the fuel cells. In this case, all parts of the fuel cell are constructed with solid materials, because the oxide ion conductor is a solid, therefore simplifying the structure. Further, liquid leakage is prevented from occurring, and hence the number of times needed for maintenance operation is extremely reduced.
Examples of the oxide ion conductor include stabilized ZrO2 doped with about 8 mole % of Y2O3 (YSZ), fluorite type oxide such as CeO2 doped with about 10 mole % of Sm2O3 (SDC), and perovskite type oxide such as La0.9Sr0.1Ga0.8Mg0.2O3 and BaTh0.9Gd0.1O3 and so on.
A high temperature of about 1,000° C. is required in order to quickly migrate the oxide ion through the oxide as described above to increase the oxide ion conductivity. Therefore, it is necessary to raise the operation temperature in the fuel cell.
In order to satisfactorily operate the fuel cell in a middle temperature region of about 500° C. to 800° C., an electrolyte preferably composed of a composite oxide (apatite type oxide) of rare earth element and Si is disclosed in Japanese Laid-Open Patent Publication Nos. 8-208333 and 11-71169. The apatite type oxide exhibits satisfactory oxide ion conductivity in the temperature region as described above.
In order to further improve the operation of the fuel cell, suggestions have been made in Japanese Laid-Open Patent Publication Nos. 8-208333 and 11-71169 to further improve the oxide ion conductivity of the apatite type oxide. Japanese Laid-Open Patent Publication No. 11-130595 discloses that the oxide ion conduction is made anisotropic by allowing the apatite type oxide to be single crystal, and thus the oxide ion conductivity is increased along with a specified plane or in a specified direction. Meanwhile, Japanese Laid-Open Patent Publication No. 2002-252005 suggests that the Si site of apatite type oxide is substituted with Ge.
However, particularly when the oxide ion conduction of the electrolyte is anisotropic, the oxide ion conductivity of the entire electrolyte-electrode assembly is not significantly improved, even when the electrolyte-electrode assembly is produced by providing the anode and the cathode on respective end surfaces of the electrolyte. In other words, the power generation performance of the fuel cell is not significantly improved even when the fuel cell is constructed by using the electrolyte which has anisotropic oxide ion conductivity.